(1) Field of the Invention
This invention generally relates to the classification of an object by means of energy received from the object and more specifically to a method and apparatus for classifying objects into physical categories on the basis of a Doppler spread signals received from the object.
(2) Description of the Prior Art
There are many applications where it is desirable to obtain, at one site, information about an object at a remote site. For example, radar systems at one site often obtain bearing, range, altitude and flight path information for aircraft. Sonar systems on submarines can provide bearing, range, depth and trajectory information related to another object in the water such as another submarine.
Prior art systems often transmit an energy pulse from one site toward the remote object as part of the process for obtaining information about the object. These energy pulses can take the form of acoustic pulses in sonar systems or electromagnetic pulses in radar systems. For purposes of this invention, the structure and operation of radar and sonar systems are closely analogous. The structure of one can readily be adapted to the other by persons of ordinary skill in the art. For brevity, the following discussion is limited to the application of this invention in sonar systems.
An active acoustic sonar system, basically, transmits one or a series of acoustic signals into the sea and extracts certain information about the target from the returned echo or echoes received between transmitted pulses. Passive sonar systems generally receive signals from the target itself. In either form, the received information often identifies many physical and operational characteristics of the target. The reliability of information obtained from sonar systems of either type depends to a large extent on the structure and motion of the target and the medium through which the energy passes, such as temperature, density, salinity and other sea water characteristics in the case of underwater sonar systems.
Often the ability of such systems to extract accurate range and Doppler information is degraded by a phenomena called Deppler spreading,w hich can be caused by the target and/or by the medium through which an acoustic, radar or other energy wave travels. Doppler spreading can occur as a result of the changing orientation of a target, the propeller on stern aspect targets, the interference from scattering from the target, fluctuations caused by the medium and physical effects caused by motion and vibration of the sonar system itself.
My U.S. Pat. No. 5,091,890 discloses a method for extracting target range and Doppler information from such Doppler spread signals by the use of a fourth order cumulant. In accordance with that disclosure, an active sonar system transmits a series of acoustic pulses into the medium where a target resides. In one specific embodiment, received reflected sonar echoes are matched with the transmitted pulses to determine the fourth order cumulant and the fourth order cumulant spectrum of the incoming information. Other methods can be used to derive the fourth order cumulant for the received information in passive sonar systems, radar systems and the like. The specifically disclosed system produces target range and Doppler information from which motion characteristics can be determined. This information is unaffected by Doppler spreading.
This particular system and other prior systems provide range and related information with differing degrees of accuracy. However, none of the systems disclose or suggest any apparatus by which such energy from a target can be used to classify a target into some physical category.